Polarization diversity can be used in communication systems for providing two parallel communication channels having orthogonal polarizations over the same link, thus doubling the link capacity. Separate and independent signals are transmitted using the two orthogonal polarizations. Despite the orthogonality of the channels, however, some interference between the signals occurs almost inevitably. In order to reduce the effects of this interference, the receiver may comprise a cross-polarization interference canceller (XPIC), which processes and combines the two signals in order to recover the original, independent signals.
A variety of XPIC circuits are known in the art. XPIC circuits are described, for example, in U.S. Pat. Nos. 4,914,676, 5,920,595, 5,710,799, in European Patent Application EP 1365519 A1, and in PCT Patent Application WO 00/77952 A1, whose disclosures are all incorporated herein by reference.
In some applications, the interference cancellation process varies the phase of the interference signal. For example, U.S. Pat. No. 6,236,263, whose disclosure is incorporated herein by reference, describes a demodulator with a cross-polarization interference canceling function for canceling interference of cross polarization in the main polarization. The demodulator includes a demodulating unit for demodulating a baseband signal of the main polarization and a phase control unit which controls the phase of an interference signal of cross polarization, based upon an error in the demodulated signal. An interference cancellation unit cancels an interference signal component from the demodulated signal of the main polarization.